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Seaweed is a loose colloquial term encompassing
macroscopic, multicellular, benthicmarinealgae. The term includes some members of the red, brown and green algae. Seaweeds can also be classified by
use (as food, medicine, fertilizer, industrial, etc.).

Taxonomy

A seaweed may belong to one of several groups of multicellular
algae: the red algae,
green algae, and brown algae. As these three groups are not
thought to have a common multicellular ancestor, the seaweeds are a
paraphyletic group. In addition, some
tuft-forming bluegreen algae (Cyanobacteria) are sometimes considered as
seaweeds — "seaweed" is a colloquial term and lacks a formal
definition.

Ecology

Two specific environmental requirements dominate seaweed ecology. These are the presence of seawater (or at
least brackish water) and the
presence of light sufficient to drive photosynthesis. Another common requirement is
a firm attachment point. As a result, seaweeds most commonly
inhabit the littoral zone and within that
zone more frequently on rocky shores than on sand or shingle.
Seaweeds occupy a wide range of ecological niches. The highest
elevation is only wetted by the tops of sea spray, the lowest is
several meters deep. In some areas, littoral seaweeds can extend
several miles out to sea. The limiting factor in such cases is
sunlight availability. The deepest living seaweeds are the various
kelps.

A number of species such as Sargassum have adapted to a fully planktonic niche and are free-floating, depending
on gas-filled sacs to maintain an acceptable depth.

Others have adapted to live in tidal rock
pools. In this niche seaweeds must withstand rapidly changing
temperature and salinity and even
occasional drying.

Uses

Seaweed has a variety of purposes, for which it is farmed or foraged from the wild.

In Asia, Zicai (紫菜) (in China), gim (in Korea) and nori (in Japan) are sheets of dried Porphyra used in soups or to wrap sushi. Chondrus
crispus (commonly known as Irish
moss or carrageenan moss) is another
red alga used in producing various food additives, along with
Kappaphycus and various gigartinoid
seaweeds. Porphyra is a red alga used in
Wales to make laver.Laverbread, made from oats
and the laver, is a popular dish there.
Affectionately called "Dulce" in northern Belize, seaweeds are
mixed with milk, nutmeg, cinnamon, and vanilla to make a common
beverage.

Seaweeds are also harvested or cultivated for the extraction of
alginate, agar and
carrageenan, gelatinous substances
collectively known as hydrocolloids or
phycocolloids. Hydrocolloids have attained commercial significance
as food additives. The food industry exploits their gelling,
water-retention, emulsifying and other physical properties. Agar is
used in foods such as confectionery, meat and poultry products,
desserts and beverages and moulded foods. Carrageenan is used in
salad dressings and sauces, dietetic foods, and as a preservative
in meat and fish products, dairy items and baked goods.

Medicine

Alginates are used in wound dressings, and production of dental
moulds. In microbiology research, agar
is extensively used as culture medium.

Seaweed extract is used in some diet pills. Other seaweed pills
exploit the same effect as gastric
banding, expanding in the stomach to make the body feel more
full.

Other uses

Other seaweeds may be used as fertilizer. Seaweed is currently under
consideration as a potential source of bioethanol. Seaweed is an ingredient in some
toothpaste, cosmetics and paints.

Alginates enjoy many of the same uses as carrageenan, and are used
in industrial products such as paper coatings, adhesives, dyes,
gels, explosives and in processes such as paper sizing, textile
printing, hydro-mulching and drilling.